如何在iOS中将UIImage保存为渐进式JPEG格式？

Question

在iOS中将UIImage另存为JPEG，我使用UIImageJPEGRepresentation，但是它不采用压缩比以外的选项。我希望将UIImage保存为progressive JPEG格式，有没有简单的方法可以做到这一点？

看起来像在OS X中有一个NSImageProgressive选项将NSImage保存为渐进格式。

Answer 1

我想你可以使用的ImageIO框架，这样做：

#import <UIKit/UIKit.h> 
#import <ImageIO/ImageIO.h> 
#import <MobileCoreServices/MobileCoreServices.h> 
#import "AppDelegate.h" 

int main(int argc, char *argv[]) 
{ 
    @autoreleasepool { 
     UIImage *sourceImage = [UIImage imageNamed:@"test.jpg"]; 

     CFURLRef url = CFURLCreateWithString(NULL, CFSTR("file:///tmp/progressive.jpg"), NULL); 
     CGImageDestinationRef destination = CGImageDestinationCreateWithURL(url, kUTTypeJPEG, 1, NULL); 
     CFRelease(url); 

     NSDictionary *jfifProperties = [NSDictionary dictionaryWithObjectsAndKeys: 
      (__bridge id)kCFBooleanTrue, kCGImagePropertyJFIFIsProgressive, 
      nil]; 

     NSDictionary *properties = [NSDictionary dictionaryWithObjectsAndKeys: 
      [NSNumber numberWithFloat:.6], kCGImageDestinationLossyCompressionQuality, 
      jfifProperties, kCGImagePropertyJFIFDictionary, 
      nil]; 

     CGImageDestinationAddImage(destination, sourceImage.CGImage, (__bridge CFDictionaryRef)properties); 
     CGImageDestinationFinalize(destination); 
     CFRelease(destination); 

     return 0; 
    } 
} 

Preview.app说，输出文件是渐进的，而ImageMagick的identify命令说，它有“隔行扫描：JPEG”。

Answer 2

此代码在设备上工作 - 关键是要指定所有的密度值（注意其使用新文本语法）：

- (UIImage *)imager 
{ 
    UIImage *object = [UIImage imageNamed:@"Untitled.jpg"]; 
    NSString *str = [[self applicationDocumentsDirectory] stringByAppendingPathComponent:@"Tester.jpg"]; 
    NSURL *url = [[NSURL alloc] initFileURLWithPath:str]; 

    CGImageDestinationRef destination = CGImageDestinationCreateWithURL((__bridge CFURLRef)url, kUTTypeJPEG, 1, NULL); 

    NSDictionary *jfifProperties = [NSDictionary dictionaryWithObjectsAndKeys: 
            @72, kCGImagePropertyJFIFXDensity, 
            @72, kCGImagePropertyJFIFYDensity, 
            @1, kCGImagePropertyJFIFDensityUnit, 
            nil]; 

    NSDictionary *properties = [NSDictionary dictionaryWithObjectsAndKeys: 
           [NSNumber numberWithFloat:.5f], kCGImageDestinationLossyCompressionQuality, 
           jfifProperties, kCGImagePropertyJFIFDictionary, 
           nil]; 

    CGImageDestinationAddImage(destination, ((UIImage*)object).CGImage, (__bridge CFDictionaryRef)properties); 
    CGImageDestinationFinalize(destination); 
    CFRelease(destination); 

    return [UIImage imageWithContentsOfFile:str]; 
}